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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
        <div>
                <p>圆的半径：<input id="r" type="text"></p>
                <p>圆的周长：<input id="cir" type="text"></p>
                <p>圆的面积：<input id="area" type="text"></p>
            </div>
            <script>
              var r=prompt("请输入圆的半径");
              r=parseFloat(r) && Number(r);
              if(!isNaN(r))
              {
                  var cir=2*Math.PI*r;
                  var area=Math.PI*r*r;
                  document.getElementById('r').value=r;
                  document.getElementById('cir').value=cir.toFixed(2);
                  document.getElementById('area').value=area.toFixed(2);
              }
              else
              {
                  alert('请输入正确的数字');
              }
            </script>
        
</body>
</html>